Home canning is an excellent way to preserve garden produce and share it with family and friends, but it can be risky—or even deadly—if not done correctly and safely. The potential culprit is botulism.
Although it’s a rare occurrence, botulism is a serious illness caused by a bacterium toxin, Clostridium botulinum, that attacks the body’s nerves. It can cause difficulty breathing, muscle paralysis, and even death. Clostridium botulinum is commonly found in soil, on raw fruits and vegetables, on meat and fish, and many other foods and surfaces. Improperly home-canned, preserved, or fermented foods can provide the right conditions for the bacteria to make the toxin. One cannot see, smell or taste the toxin, yet if ingested, even a small amount can be deadly.
To avoid concerns about botulism, steer clear of these canning mistakes .
Making Up Your Own Recipe
Use only up-to-date, scientifically tested recipes approved by the USDA and follow directions exactly. Good sources are The Complete Canning Guide, the National Center for Home Food Preservation, So Easy to Preserve, and land-grant university publications. Without scientific testing, there is no way to know how long the product needs to be processed to be safe.
Adding Extra Starch, Flour or Other Thickeners
Starches slow the rate of heat penetration into the product and can result in under processing. Clear Jel® is recommended as a thickener for canned pie fillings. If a product is not as thick as desired going into the jar, it can be thickened at the time of use.
Adding Extra Onions, Chilies, Peppers, or Other Vegetables to Salsas or Sauces
Tested recipes specify the quantity of vegetables allowed to keep the salsa or sauce within the safe pH range. Most vegetables are low-acid and adding extra vegetables will dilute the acidity and result in an unsafe product. While it is dangerous to add more vegetables to salsa and sauce recipes, fewer may be used for a milder flavor. Extra ingredients can be added at the time of use.
Canning by Electric Pressure Cooker, Oven, Open Kettle, Microwave, or Dishwasher
Boiling water bath or pressure are the only approved canning methods. Any other method should be avoided as foods will be under processed and unsafe. Heat is conducted by air in an oven which is less efficient than water or steam. As a result product temperature never exceeds the boiling point; jar breakage is also a possibility. Open-kettle canning, placing hot food in jars and sealing with no further heat treatment, has been considered an unsafe home canning practice since the 1980’s due to insufficient heat to destroy bacteria . Microwaved food reaches 212°F but heating is not uniform; in additon, there is a danger of jar explosion within the microwave oven or as food is being removed . The water temperature of the dishwasher’s cleaning and rinsing cyles is far below that required to kill harmful microorganisms. While electric pressure cookers use pressure for cooking, they do not meet the appliance standards for home pressure canning, even if marketed as safe for pressure canning. Washington State Extension explains all the reasons why electric pressure cookers should not be used for home canning in Canning in Electric Pressure Cookers.
Not Making Altitude Adjustments
The temperature at which water boils is affected by barometric pressure which is reduced with elevation or altitude. When water doesn’t reach the normal boiling temperature, undesired pathogens may survive the canning process. These pathogens could multiply in the canned food and cause sickness. Processing times and temperatures for recipes in most canning resources are based on canning at an elevation of 1,000 feet above sea level or lower. When canning at a higher elevation, one must add more pounds of pressure for pressure canning and more processing time for water bath canning. See charts prepared by South Dakota State University Extension for altitude adjustments.
Not Venting the Pressure Canner
Trapped air inside a pressure canner lowers the temperature obtained for a given pressure and results in under processing. To be safe, the USDA recommends that all pressure canners be vented 10 minutes before they are pressurized.
To vent a canner, leave the vent pipe (steam vent) uncovered (or manually open the petcock on some older models) after you fill the canner and lock the canner lid in place. Heat the canner on high until the water boils and generates steam that can be seen escaping through the open vent pipe or petcock. When a visible funnel-shape of steam is continuously escaping the canner, set a timer for 10 minutes. After 10 minutes of continuous steam, you can close the petcock or place the counterweight or weighted gauge over the vent pipe to begin pressurizing the canner. 
Failure to Acidify Canned Tomatoes
Because the pH of tomatoes is an unknown, the USDA recommends that bottled lemon juice be used to lower the pH of the product to be unquestionably safe.
Rushing the Pressure Canner Cool-Down Time
The natural cool-down of the canner is part of the tested processing time. Hurrying this process will result in under-processed food and siphoning of liquid from the jars. It may also cause jar breakage.
Allowing “Hot Pack” Foods to Cool in Jars before Processing
Processing times are based on the food being hot at the beginning of the processing. Foods not starting hot could be under processed. Further, the rule, “hot foods hot and cold foods cold” applies; when foods are held between 40 and 140°F, bacteria can grow rapidly.
Processing Low-Acid Foods in a Water Bath
Canning low-acid foods requires special care. This includes red meats, fish, poultry and all vegetables (except for acidified tomatoes). Low-acid foods can support the production of the deadly botulism toxin if these foods are not processed properly in a pressure canner. A pressure canner heats food to high temperatures (240-250 degrees F or higher) and destroys the spores that produce the botulism toxin. A boiling water bath canner, which can be used for canning pickles or fruit, heats food to boiling temperature (212 F), which is not high enough to ensure safety for canning vegetables and other low-acid foods.
Home canning is perfectly safe but needs to be done correctly. If you are new to canning (or need a refresher), check out Safe Home Canning Basics, by Minnesota University Extension to learn about preventing botulism and other key issues to be aware of when preserving food by home canning.